1. Field of the Invention
Aspects of the present invention relate to a multiple-input multiple-output (MIMO) system, and more particularly, to a codebook of precoding matrices for use in MIMO systems and a method of generating a codebook for use in such MIMO systems.
2. Description of the Related Art
Currently, wireless communication technologies for providing a variety of multimedia services in wireless communication environments are expanding. High-speed data transmission is needed to provide high quality multimedia services in wireless communication systems. Accordingly, various research has been conducted to support high-speed data transmission in such wireless communication systems. A recent proposal to achieve high-speed data transmission relates to the use of multiple antennas at both the transmitter and the receiver, known as multiple-input multiple-output (MIMO) systems.
MIMO technology offers significant increases in channel capacities with limited frequency resources and in data transmission rates by using multiple antennas at both the transmitter and the receiver. In such MIMO systems, a number of antennas are used when scattering conditions are substantial, and theoretically, MIMO systems provide channel capacities proportional to the number of antennas. Such MIMO technology can serve as an important component of the next generation mobile communication systems, such as 3rd Generation Partnership Project (3GPP), Super 3G (or 3G Long Term Evolution “LTE”), 3GPP2 and upcoming 4G systems, particularly, for the downlink from a single base station to multiple user equipments.
However, when MIMO technology is deployed, the physical space and area for installing antennas may be limited. Communication systems using MIMO technology are highly affected by spacing between antennas. Specifically, as the spacing between antennas becomes smaller, higher correlation between wireless channels can be generated. Particularly, when antennas have the same polarization, higher correlation between wireless channels can be generated. Correlation generated between wireless channels reduces reliability for data communication and also reduces data transmission rates.
Accordingly, various methods of using the polarization direction of antennas are needed in order to reduce an area for installing multiple antennas and also to increase channel capacities. When multi-polarized antennas are used in MIMO systems, Correlation between wireless channels can be reduced.
Coding operations, referred to as a precoding, are needed to effectively transmit data, via wireless channels, in MIMO systems to maximize system performance and capacity. Precoding represents multi-layer beamforming in which a transmission signal (data) is emitted from each of the antennas in accordance with a data precoding rule, i.e., appropriate phase (and gain) weighting such that the signal power is maximized at the receiver input and the multipath fading effect is minimized. The weight can be expressed in terms of a precoding matrix (i.e., a set of beam-forming vectors) and is selected from a set of precoding matrices in a codebook.
Currently, there are various types of codebooks designed for the particular cases of single-polarized MIMO schemes where the polarization of antennas is single-polarization. However, no effective codebook has been offered in association with cases of multi-polarized MIMO schemes where the polarization of antennas is multi-polarization. Existing codebooks designed for single-polarized MIMO schemes cannot be optimized for multi-polarized MIMO schemes.
Accordingly, there is a need for a method and apparatus for generating a codebook for a MIMO system with low complexity and excellent performance, even when the polarization of antennas is multi-polarization.